Spot Reaction Experiments Part : 'FI
Protective Layer Effects FRITZ FEIGL
Laboratorio Central da Produ@o Mineral, Ministerio da Agricultura, Rio de Janeiro, B r a d
(Translated by Ralph E. Oesper, University of Cincinnati)
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HEN sohds participate in chemical reactions the First, the coating must be as coherent as possible and reachon place or starts on the free surface. must adhere firmly to the compound to be protected. . takes ' Consequently, the reaction is impeded if the surface Secondly, i t must be easy t6 detect the differences in of the solid is coated with a coherent layer or film of the chemical behavior of the protective layer and the resistant material. Such protective layers (galvaniz- protected compound. Consequently, the quick recing, anodic oxidation, painting) are often applied in- ognition of a protective layer effect demands redustrially to protect metal surfaces against corrosion actions that are accompanied by definite color changes. Theoretically, the complete coating of a material by water and atmospheric agencies or attack by chemicals. Sometimes surface protective layers of oxide with no more than a few layers of molecules of a reare formed by heating metals in the air. Such a m s sistant compound would suffice to produce a protective are responsible for the "passivity" of metals, which layer. In practice, such layers on impregnated papers consist of much thicker coatings, which are never then show little or no reaction toward acids. The pure metal cannot be distinguished optically perfectly continuous. The imperfections in the coating from passive metal. This shows that even an extremely arise from the fact that when the solid penetrates and thin layer of oxide is adequate for protection. Con- is dispersed among the capillariesof the paper, irregularisequently, i t seems logical to expect that the surfaces ties and blank spaces are left. The reagents pass of crystalline or amorphous precipitates may also be through these into the interior of the coating and undercoated with a protective layer and thus shielded against go chemical reaction there. Consequently, as a rule, the coating is only partial; nevertheless, the differcertain chemical attacks. Experiments designed to detect the presence of a ences in the reaction rate of the protected and unprotective layer and to demonstrate the protective protected areas of the paper are sufficient to permit effect are always cumbersome, if made with precipitates very sensitive tests. The best effects are obtained when the protective produced in test tubes. In contrast, protective layer effects are easily and distinctly demonstrated with layer is formed by direct reaction with a material imsolids on paper. With their aid i t is possible to show bedded in the capillaries of the paper, because the that the formation of protective layers need not be coating then fits very snugly. Indifferent materials, limited to the coating of metals, in which there is a however, can also produce protective layers. Exgreat disproportion between the quantity of the metal cellent effects are provided by gelatinous materials and the protector, but that structures possessing ex- since they can form a smooth film or coating over the tensive surface and slight thickness and weight are also surfaces of solids. In contrast, compounds that crystallize well, and so cannot cling tightly to a given susceptible to coating, and on all sides. In addition to this fact, which is very instructive in surface, usually do not provide good protection. The understanding the fundamentals of surface coatings, the following experiments illustrate these points. formation of protective layers on materials highly dis13. Protectine Layer Effect of Palladium Dimethylpersed in capillaries also has a practical analytical glyoxime on Nickel Dimethylglyoxime. If a drop of a significance. It is possible, by means of spot reactions, neutral solution of a palladium salt is placed on paper sometimes to make sensitive tests to detect materials impregnated with nickel dimethylglyoxime, the red nickel salt is converted into the yellow palladium salt that are capable of providing a protective layer. The basic principle of the procedure for detecting a of dimethylglyoxime: protective layer effect by spot reactions is this: a paper Ni(DH)r + Pd++ = Ni++ Pd (DH)n impregnated with a water-insoluble material is spotted (DH = dirnethylglyoxime radical) with a drop of a solution containing a reagent which, of itself or by reaction with the compound in the paper, Accordingly, considerable quantities of palladium form can form a protective layer. Then the paper is treated a yellow fleck on the red paper. However, these salts with a reagent which acts only on the unprotected com- differnot only in color but also in their behavior toward dilute mineral acids. The nickel salt dissolves impound, leaving the area of the fleck unchanged. Two conditions must be fulfilled if the protective mediately, whereas the palladium salt is acid-resistant. layer effect is to be of value in analytical procedures. Consequently, if Ni-dimethylglyoxime paper is spotted 298
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with a strong Pd++ solution and then is placed in acid, the red paper will be decolorized except on the spotted area, where yellow Pd-dimethylglyoxime will be left. The results are quite different if very dilute Pd++ solutions are used. No yellow spot will be visible on the reagent paper, but when it is soaked in dilute acid, an intense red spot will remain while the rest of the paper turns white. The explanation of this phenomenon is this: the Nidimethylglyoxime within the capillaries of the paper reacts &st on its free surface with Pd++ ions and thus covers itself with a layer of Pd-dimethylglyoxime. This layer or coating prevents or retards the further penetration of the palladium solution. If this solution is dilute, the Ni-dimethylglyoxime will be covered only superficially and the unchanged material under the coating will be shielded against the action of acids. The red spot that is left after treating Ni-dimethylglyoxime paper with dilute palladium solution, followed by treatment with acid, is therefore merely Nidimethylglyoxime, protected against solution in acid by a thm invisible layer of Pd-dimethylglyoxime. The foregoing protective layer effect can be accomplished by even minute quantities of Pd-dimethlyglyoxime. Its potency probably arises from the fact that the palladium and nickel salts of dimethlyglyoxime are isomorphous. Therefore, mixed crystals are formed on the surface of the Ni-dimethlyglyoxime. Consequently, the fit of the coating is the best imaginable, and accordingly it protects the underlying nickel salts to an extremely high degree. Procedure: Filter paper is soaked in one per cent alcoholic solution of dimethlyglyoxime and dried in a blast of warm air. Several strips of this paper are then soaked in 2 N ammoniacal nickel nitrate solution. Red Ni-dimethylglyoxime is thus precipitated in the capillaries of the paper. The reagent paper is washed with water, then alcohol, and then dried in a blast of warm air. It will keep for several weeks. On aging, the Ni-dimethylglyoxime recrystallizes slowly and part of it dusts away, so that the paper loses some of its original activity. One drop of 0.01 per cent PdClz solution is placed on the reagent paper. After the drop has been completely absorbed, the paper is placed in dilute hydrochloric acid. The red paper turns white immediately, except the spotted area, which remains red. The paper is then soaked in water to wash out the acid. For comparison, paper impregnated with dimethylglyoxime alone is spotted with the palladium chloride solution and then soaked in dilute hydrochloric acid. A yellow spot of insoluble Pd-dimethylglyoxime is left. The drop tests are repeated on both reagent papers with progressive dilutions of the PdClz solution. It will be found that dilute solutions giving no positive response for palladium on dimethylglyoxime paper nor with other reagents will, after the acid treatment, leave distinct red spots on Ni-dimethylglyoxime paper. This test for palladium will reveal as little as 0.05 y of Pd in one drop. The test is definitely specific since,
under these conditions, no other metals react or decrease the sensitivity.' 14. Protective Layer Effect of Thallium Iodide and Lead Iodide on Underlying Suljides. When flter paper impregnated with TlzSor PbS is spotted with a drop of an ether solution of iodine, the corresponding iodide is formed. If considerable quantities of iodine are involved, the reaction can be detected directly by the color change from brown or black (sulfidP) to yellow (iodide). The iodine solution can be gradually diluted until it is no longer possible to see any formation of TI1 or PbIe, which occurs with even small quantities of iodine. If, however, the sulfide paper with the invisible iodide spot is placed in a solution of hydrogen peroxide the paper turns white quickly, except where the dilute iodine solution was applied. Brown flecks are left there. The phenomenon can be explained as a protective layer effect. The sulfides, highly dispersed in the capillaries of the paper, are superficially converted by iodine into the corresponding iodides. If only small quantities of iodine react, not more than a thin layer of iodide is formed. This is capable of protecting the underlying unaltered sulfide from the action of hydrogen peroxide. The latter oxidizes the bare dark sulfide to colorless sulfate. In contrast, it has no effect on the portions shielded by the iodide, or at least the oxidation proceeds only slowly. Consequently, the spot picture produced in the following experiment demonstrates the protective layer effect of quantities of T1I or PbIz, that are so small that, of themselves, they are not visible. Procedure: Sensitive papers are prepared by soaking strips of filter paper in 0.1 N thallium carbonate or 0.1 N lead acetate solution and then placing over ammonium sulfide. The paper blackens gradually as the respective sulfides are formed. The reagent papers are then washed briefly and dried. A drop of two per cent ether solution of iodine is placed on each of the papers and the spots are held for several seconds in a blast of warm air. The conversion of the highly dispersed black sulfide into the corresponding yellow iodide will be observed. The iodine solution is now diluted with ether until no visible formation of iodide occurs when a drop is placed on the sulfide papers. The spotted papers are then placed in three per cent hydrogen peroxide solution. Within one or two minutes, the papers turn perfectly white, with the exception of the spotted areas which remain black or brown. 15. Protective Layer Effect of Fat or Parajin on the Surface of Metal Salts. The experiments described in paragraphs 13 and 14 dealt with protective layers produced by direct reaction of suitable reagents with materials finely dispersed in filter paper. Indifferent materials can also produce protective layer effects through mechanical coating of materials dispersed in paper. They are thus shut off from attack by acids or other reagents. 'FsrcL. Chmistry and Industry, 57,1161 (1938).
Dilute benzene solutions of paraffin or fat are used in these experiments. If a drop of the solution is placed on paper impregnated with TlS, HgzClr, or Ni-dimethylglyoxime, and the solvent allowed to evaporate, the impregnated material in the spotted area will be coated with a thin invisible lilm. Consequently, the protected T1S or Ni-dimethylglyoxime is not dissolved by acids, and brown or red spots are left when the rest of the reagent paper is decolorized. The spotted areas on the HgzClz paper have lost the ability to react with dilute ammonia (HgzC12 2NHs = Hg HgNHzCl NHCI), or the reaction is very slow. A white or light gray spot is left, surrounded by black or gray where the free mercury has deposited. Procedure: The preparation of thallium sulfide paper was described in paragraph 14; of the Ni-diiethylglyoxime paper in paragraph 13. The mercurous chloride paper is prepared hy soaking lilter paper in Hgz(NO& solution and then in dilute hydrochloric acid. The paper, which is thus impregnated with HgzClz,is washed with water and dried in a blast of warm air, or the water is removed by soaking in alcohol. This reagent paper should be stored in the dark. One drop of a two per cent solution of f a t or paraffin
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in benzene is placed on paper impregnated with TlzS or Ni-dimethylglyoxime. After the solvent has evaporated the paper is placed in dilute acid. If calomel paper is used it should be immersed in 0.3 per cent ammonia water after the solvent has evaporated. As soon as the spot picture of the protective layer effect has developed, the paper is placed in water to prevent the slow penetration of the reagent with consequent destruction of the spot picture. The experiments should be repeated with drops of the fat or paraffin solution that have been diluted to as much as 20 times their original volume by the addition of benzene. This protective layer effect will reveal smaller quantities of fat than the common "grease spot test." In the latter, a drop of a fat solution is placed on thin tissue or cigarette paper. After the solvent has volatilized, a grease spot is left which, because of its translucence, stands out on the unaltered paper. A comparison of these two methods will demonstrate that the protective layer effect is a far more sensitive test. Consequently, this procedure can be used to detect even small quantities of fat extracted from the specimen by benzene.
Part V: Solvent Effects2
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F CHEMICAL reactions take place in solutions of (ether, alcohol, etc.) are believed to contain rather the participants, or if a t least one of the reactants stable iodine-solvates. These are complex compounds is dissolved, the solvent also becomes a factor in the re- in which iodine molecules are coordinated through action. This statement should be taken in the sense auxiliary valences on certain atoms of the molecules of that the solvent, first of all, provides the reaction the solvent. The following coordination formulas medium in which the molecules or ions move and so can be written for the ether solvate of iodine: come in contact with the molecules and ions of the other reactants. Furthermore, solutions of all kinds always contain loose combinations ("solvates") of the solvent with the solute. Strictly speaking, then, the reactants (actual) are not molecules (ions) of the dis- Formula (I) merely designates that the iodine molecule solved material, but rather compounds of the solvent. is bound to the oxygen atom of the ether molecule. The solvates can always be regarded as addition com- Formula (11) shows the ether molecule bound to both pounds of molecules (ions) of the solute with molecules of the atoms of the iodine molecule by auxiliary vaof the solvent. It therefore seems logical that different lences. The original mutual binding of the atoms of solvates should exhibit different activities. This signi- the iodine molecule is thus loosened. If this view is fies that the solvent may influence the chemical re- accepted, the iodine molecule, which is combined actions of the materials dissolved in it. Examples of with the solvent molecules in the brown solutions, should be activated, because the union between the this effect are shown in the following experiments. 16. Dissimilarily i n the Activily of Iodine i n Carbon iodine then corresponds more closely to the atomic Disuljide and i n Ether. When iodine is dissolved in rather than to the molecular condition. The following experiments will actually demonorganic liquids violet or brown @~tions are formed. The violet solutions (CS2,benzene, etc.), whose color is strate that iodine in brown ether solutions is more relike that of iodine vapor, are assumed to contain molecu- active toward metallic copper, silver cyanide, and silver lar iodine, or iodine loosely combined with molecules cyanate, than equal concentrations in a violet carbon of the solvent. In contrast. the brown iodine solutions disulfide solution. This signifies that the velocitv of the reactions
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a FEIGL, "Specific and Special Reactions." Tmnrlated by R. E. Ocspcr. Nordeman Publishing Co., New York, 1940, p. 64.
Cu f I,
=
Cud2 or AgCN
+ L = AgI f CNI
is affected by the solvent. The same is true of the reaction with AgCNO. Solutions of iodine in ether and in carbon disulfide also react quite differently toward calomel (Hg2CI2), which is not soluble in organic liquids. In both cases, iodine is consumed by the reaction The formation of iodide, and consequently the discharge of the color, is almost instantaneous with the brown ether solution, while it is slow in the case of the violet carbon disulfide solution. The rapid reaction involves not only the activation of the iodine, through solvate formation, but the additional favoring circumstance that both products of the reaction (HgIz and HgCI2), are quite soluble in ether. Accordingly the reactive surface of the solid product is not coated by the iodide formed, as in the reactions of iodine with metallic copper or silver cyanate. The surface of the calomel always remains bare and hence susceptible to attack by the iodine. Procedure: Five milliliters of brown, one per cent solution of iodine in carbon disulfide are placed in a small test tube and five milliliters of violet, one per cent solution of iodine in ether are placed in a second tube. Strips of copper foil (1 X 2 an.) are dropped into each of the solutions and shaken from time to time. After about 15 minutes the brown solution will have lost all its color and white Cu& will have been deposited on the copper foil. In contrast, the violet solution will be practically unchanged, and will not lose its color for several hours. The experiment is repeated with one gram of powdered copper in place of the foil. The brown solution is decolorized within one to two minutes, because of the extensive active surface. The violet solution requires about 20 minutes for the complete discharge of its color. Approximately equal quantities of freshly precipitated and dried silver cyanide or silver cyanate are placed in adjacent depressions of a spot plate. One depression is then filled to the brim with the violet solution of iodine, the other with the brown solution. On stirring with a wooden splint, the brown solution in contact with AgCNO loses its color almost immediately, whereas the violet solution requires 24 hours. When silver cyanide is used, the difference is not so striking: five minutes for the brown solution, 10 minutes for the violet. The action of calomel with brown and violet iodine solutions can be compared as follows: one-gram portions of HgC12are placed in two small test tubes. One portion is shaken with 5 ml. of the ether solution, the other with 5 ml. of the carbon disulfide solution. The brown ether solution loses its color completely in about 10 seconds, whereas the violet solution apparently remains unchanged. The latter requires about two minutes of shaking before the iodine is consumed. 17. Dgerences in the Activity of Sulfur in organic Solvents. Sulfur dissolves to different extents in
various organic liquids. The best solvent is carbon disulfide, which a t room temperature dissolves all crystalline modifications of sulfur to about 30 per cent. It has no action on amorphous sulfur. On the other hand, all forms of sulfur are soluble in pyridine, but only to the extent of about 2.5 per cent a t room temperature. The solubility of sulfur in other organic solvents is seldom more than this. Sulfur dissolved in organic liquids forms sulfides with seme metals; mercury, in particular, reacts very quickly. Accordingly, dissolved sulfur reacts similarly, although far more slowly, than sulfur vapor or molten sulfur. It is remarkable that despite the high solubility of sulfur in carbon disulfide, its activity toward silver is considerably less than when dissolved in pyridine. Consequently, the addition reaction of dissolved sulfur on metallic silver (2Ag S = AgzS) does not depend upon the quantity of sulfur in solution. The solvent has a definite influence on the rate of formation of sulfide. Solutions of sulfur in other organic liquids, when tested a t the respective boiling points and allowed to react for five minutes behave as follows: carbon tetrachloride (76%) and ether (34°C.) act like carbon disulfide (46°C.); benzene (80°C.) and toluene (lll°C.) act like pyridine (115'C.) solutions. In considering the behavior of CC4 solutions, i t is probable that something besides a heat effect is involved in the action of the benzene and toluene solutions. The nature of the solvent seems also to play a part in the formation of silver sulfide. This is clearly indicated when saturated solutions of sulfur in benzene and thiophene are allowed to react on metallic silver a t 40°C. Within 10 minutes the surface of the silver is completely covered with Ag2Sin the case of the thiophene solution, whereas no change is visible with the benzene solution. Procedure: Three milliliters of saturated pyridine and carbon disuEde solutions of sulfur are placed in separate test tubes. Pieces of silver foil (0.5 X 1 an.) are placed in each and shaken occasionally. The silver in the pyridine solution will show a light brown color within five minutes, while the metal in the carbon disulfide solution shows no change for a long time. At room temperature a very slight deposit of AgzS appears only after 24 hours in the CS2 solution. In the same period, the foil in the pyridine solution turns completely black. If the experiment is repeated, with the test tubes in water a t 35OC., the silver in the pyridine solution will be perfectly black within 30 minutes, but no change will be observed on the metal immersed in the carbon disulfide solution. The activity of sulfur dissolved in benzene and in thiophene may be compared by carrying out the tests in tubes kept in warm water a t 40°C. The sulfide formation in the thiophene solution requires about 10 minutes. At room temperature, a formation of sulfide can be seen in the benzene solution after about 12 hours. Conseauentlv. ,, the solvent is a definite factor in determining the rate a t which AgzS is formed.
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